1. Field of the Invention
The current invention relates to the field of signal equalization, particularly to enabling improved high-speed adaptive equalization.
2. Description or Related Art
Increased demand for high-speed communications services has required that economical and efficient new devices and techniques be developed to support performance increases. For example, as transmission rates climb to the 10–40 Gb/s range and beyond in modern optical networks, signal processing and conditioning techniques must be applied to filter out noise and reduce interference such as inter-symbol interference (ISI). Typical optical networks are plagued by noise and bandwidth limitations caused by polarization mode dispersion, modal dispersion, chromatic dispersion, limited component bandwidth, and/or other undesired phenomena. Such effects often cause problems such as group delay distortion, frequency-related attenuation, and/or others. Furthermore, the ISI can be time varying due to a variety of causes such as physical vibration, mechanical stresses and temperature fluctuations. Typically, optical receivers may use devices such as equalizers to improve the overall performance of such systems and minimize the error rate. However, the implementation of such devices has proven to be challenging and costly.
Equalizers based on transversal filters have been promoted as a way of removing noise and inter-symbol interference in some systems. For example, FIG. 1 (prior art) illustrates an example of a proposed transversal filter based equalizer 10 controlled by a microprocessor 50. In this example, the coefficients for the transversal filter may be set by the adaptation logic module, a microprocessor 50, based on analysis of eye monitor 30 data. However, this design requires the use of a clock 40 for the purpose of synchronizing data sampling. This type of design may fail in cases of severe distortion such as inter-symbol interference. For example, FIGS. 2a and b illustrate examples of eye patterns. For example, a typical eye monitor may sample in or near the “center” 60 of an eye pattern 70. However, systems experiencing severe interference may exhibit a “closed” eye pattern, 80. Typical eye monitors may fail in this situation.
Furthermore, previously proposed equalizers based on transversal filters have focused on minimization of an error function involving the analytical calculation of partial derivatives of the error function with respect to the coefficients of the filter. This calculation provides an analytical expression of the gradient of the error function. The coefficient values of the filter are adjusted by subtracting a scaled version of the gradient. This process of computing the gradient using the analytical expressions for the partial derivatives and then adjusting the coefficients is performed repeatedly. However, there are distinct disadvantages in approaches that involve the analytical calculation of the gradient. First, the analytical calculation may simply be impossible to perform. For example, the analytic expression of the gradient may not be available if the error function is the measured bit error rate of a communications link. Second, even if the analytic expression of the gradient is obtained, it may not be possible to actually evaluate it since all of the necessary data may not be available. For instance, the gradient associated with a digital transversal filter may be: (2*error*x(t), 2*error*x(t−T), 2*error*x(n−2T), . . . ), where x(t) denotes the input signal and T denotes the sample period of the digital filter. The delayed values of x(t) may not be readily available in a distributed transversal filter implementation.
Accordingly, it is desirable to achieve high-speed adaptive equalization that can effectively operate, even for systems experiencing severe distortion, without the need to analytically calculate partial derivatives of an error measure with respect to filter coefficients.